A. Field of the Invention
The present invention relates to detecting and compensating for a periodic disturbance in sampled data, and more particularly to detecting in-band robbed bit signaling (RBS) in a digital carrier system for adjusting corrupted frame signals to compensate for the error introduced by robbed bit signaling.
B. Description of the Related Art
Previous implementations of analog data communications terminated in digital equipment simulate the analog codec with a .mu.-law or A-law expansion and compression functions. The compression and expansion functions may be provided in software as a look-up table or a small section of firmware implementing the equations used for the .mu.-law or A-law expansion and compression functions. Robbed bit signaling (RBS) is a communication signaling system used by the digital telephone network to transmit multiple frames of data over a digital carrier system. Analog encoded data such as voice and video applications sent over the public switched telephone network and terminated on a T-1, primary rate ISDN, basic rate ISDN or other digital service may encounter increased error rates in the analog modulations over transmission lines introduced by the in-band robbed bit signaling. Nevertheless, the effect of in-band robbed bit signaling on the companded data sample typically has been ignored.
Robbed bit signaling uses the least significant bit (lsb) of every six datum, byte or octet of a transmitted frame for signaling. In a typical digital carrier system applying robbed bit signaling, the channel banks insert an in-band signaling bit into the least significant bit position of the eight bit code words in every sixth frame, and thus every sixth code word actually contains only seven bit of voice or data information such that the effective of number of bits per sample is 7 5/6.sup.th bits instead of 8 bits. The affect upon the analog pulse code modulated (PCM) data sent over the public switched telephone network results in the introduction of a periodic disturbance in the sampled data. Depending upon the exact signaling in use, once a call is established the affected insignificant bits are typically set to either a binary one or zero consistently, but it is also possible that the digital carrier system may alternate the robbed bits between zeros and ones. Moreover, since multiple network switches may be used in a communications path which is not superframe aligned with each other, there may be several, e.g., 2, 3, 4, 5, or even 6, out of every 6 octets for which the least significant bit is being used for signaling. For data communications using modulations such as ITU-T V.34, this error is compounded by the need to interpolate, and then decimate the data from the 8 kHz network rate to the sample rates used in the modulation, such as 2400, 2749, 2800, 3000, 3200, or 3429 Hz. Thus, corrupted sample data may be used several times so as to compound the associated error.
Recently, a computer modem data rate of 33.6 kbps has been achieved, however, it has been observed that often the modem rate shift up to 33.6 kbps results in an increased block error rate (Bler) such that the effective data through put is lower than if the modem had stayed at a lower rate, e.g., 31.2 kbps. Moreover, it is also possible for modem communication to be terminated due to dropped calls related to disturbances associated with the errors introduced by the robbed bits in the communication channel.
It would be desirable to detect the periodic disturbance associated with in-band signaling in a digital telephone network to identify the use of robbed bit signaling, and further provide for robbed bit signal compensation to minimize the error introduced by the robbed bit signaling, which would provide the functional advantage of higher connect rates and lower bit error rates in data communications applications. The detecting and compensating for a periodic disturbance and sample data could provide a commercial advantage for customers who do not have access to ISDN services or those who would prefer to pay a lower rate for T-1 service as opposed to primary rate ISDN service. Such error compensation would also be advantageous to customers using basic rate ISDN or primary rate ISDN because it would improve the connections to others using T-1 services or situations where the telephone call is routed through an intermediate T-1 trunk in the public switched telephone network.